Conventionally, as a purification system for purifying nitrogen oxides (herein after referred to as NOx) even under a condition where an oxygen concentration is high like an exhaust gas of a diesel engine, an exhaust gas purification system using a NOx adsorptive catalyst and another one using an urea-selective catalytic reduction (Urea-SCR) are known. However, the exhaust gas purification system using the NOx adsorptive catalyst has a problem that a huge fuel consumption loss occurs because it need to change an air-fuel ratio of an engine from lean to rich and further to stoichiometric. On the other hand, in the exhaust gas purification system using the urea-selective catalytic reduction a problem remains in a point that a development of infrastructure of an urea becomes indispensable.
Consequently, to solve these problems, systems using NOx selective reduction catalysts, to be more precise, a platinum catalyst (for example, from a 14th row in a fifth column to a 25th row in a sixth column of page 3 of Japanese patent 2909553), an iridium catalyst (for example, from a fourth row in a fifth column to a 20th row in a sixth column of page 4 of Japanese patent laid open publication Hei 6-31173), and a silver catalyst (for example, from a 35th row in a sixth column of page 4 to a 22th row in an eighth column of page 5 of Japanese patent laid open publication Hei 5-92125) instead of the NOx adsorptive catalyst and urea-selective catalytic reduction are proposed. However, in an exhaust gas purification system using the platinum catalyst if a reducing agent, for example, a hydrocarbon (hereinafter referred to as HC) is not added in a exhaust gas, a NOx purification ratio becomes low, and if an added amount of the reducing agent is increased, a temperature of the NOx selective reduction catalyst becomes out of a range of a purification temperature region due to oxidizing heat, so a system with a high purification ratio cannot be built.
In addition, in an exhaust gas purification system using the iridium catalyst a NOx purification temperature of the catalyst is high and moreover the catalyst cannot be said to be sufficient in a selectivity for a paraffin. Accordingly, in the exhaust gas purification system an exhaust gas temperature is low, so if it is applied to a diesel engine of which paraffin concentration is high in an exhaust gas, it cannot sufficiently purify the NOx in the exhaust gas.
In addition, in an exhaust gas purification system using the silver catalyst a NOx purification temperature of the catalyst is high, so if it is applied to a diesel engine of which exhaust gas temperature is low, it cannot sufficiently purify the NOx in the exhaust gas.
Consequently, a system which substitutes the exhaust gas purification systems and can sufficiently purify the NOx in the exhaust gas is desired. Conventionally, as a trial of heightening the NOx purification ratio, a exhaust gas purification system using a plasma reactor in combination with the NOx selective reduction catalyst is known (for example, from a 19th row in a left column of page 3 to a 25th row in a right column and FIG. 1 of page 5 of Japanese patent laid open publication Hei 6-99031). However, in the exhaust gas purification system, although the NOx purification ratio is heightened in the NOx selective reduction catalyst by an exhaust gas being reformed with the plasma reactor, such a problem that the NOx purification temperature becomes high does not still be solved. In addition, in applying the exhaust gas purification system to a diesel engine with a few unburned HC in an exhaust gas, although a reducing agent such as an HC needed for reforming the exhaust gas must be added to the exhaust gas, the HC must be continued to be added in order to maintain a predetermined NOx purification ratio in the purification system. Accordingly, the purification system has a problem that a loss of fuel which becomes a supply source of the HC occurs.
On the other hand, an exhaust gas purification system using the plasma reactor and NOx adsorptive catalyst in combination with the NOx selective reduction catalyst (for example, from a 34th row in a left column of page 3 to a 48th row in a right column of page 5 and FIG. 2 of Japanese patent laid open publication 2001-182525) is known.
The exhaust gas purification system is configured so that until a temperature of the NOx selective reduction catalyst reaches the NOx purification temperature (herein after simply referred to as purification temperature) after a start of an engine, the plasma reactor converts NOx other than NO2 in an exhaust gas to NO2; and the NOx adsorptive catalyst adsorbs the NO2. And after a temperature of the NOx selective reduction catalyst reaches the purification temperature, the plasma reactor is made to be off, and the NOx in the exhaust gas continuously being sent in and NO2 desorbed from NOx adsorptive catalyst are designed to be purified by the NOx selective reduction catalyst. Accordingly, the exhaust gas purification system enables the NOx to evade itself being discharged in the atmosphere after the start of the engine until the NOx selective reduction catalyst reaches the purification temperature.
However, the exhaust gas purification system needs a NOx selective reduction catalyst which can purify the NOx within an exhaust gas temperature of a diesel engine, and taking into consideration that the exhaust gas temperature of the diesel engine is comparatively low, there exists a problem that high NOx purification performance cannot be expected. Resultingly, the exhaust gas purification system cannot efficiently purify the NOx.